The most significant factor in crane and hoist safety, after structural integrity, is electrical safety.
Referenced standards support this fact, either directly or indirectly, by the amount of definition and space provided for electrical systems' controls, operations, and maintenance.
7.2.1 NEC General Requirements
Basic installation and wiring safety requirements for cranes and hoists are given in NEC Article 610. Electrical designers and maintenance personnel should thoroughly understand these requirements and their intent. Some of the more significant requirements are the following:
1. Cranes and hoists operated in hazardous (classified) locations shall7.1 conform to NEC Article 500.
2. When the crane is operated above readily combustible materials, the resistors shall7.2 be located in a well-ventilated cabinet constructed of non-combustible material and constructed such that they do not emit flames or molten metal. See the exception (and requirements) that applies to certain cabinets made of noncombustible materials.
3. Cranes and hoists operating on electrolytic cell lines have special requirements, as given in NEC 668.
a. Grounding is not required for conductive surfaces of cranes and hoists that enter the working zone of a cell line, and the parts that come in contact with an
energized cell, or attachments, shall7.3 be insulated from ground.
b. Remote controls that may introduce hazardous conditions into the cell line working zone shall7.4 employ one or more of the following:
i. An isolated and ungrounded control circuit in compliance with NEC Section 668.21(a);
ii. A non-conductive rope operator;
iii. A pendant pushbutton with either non-conductive support and surfaces or ungrounded exposed surfaces; or
iv. A radio.
7.2.2 Disconnecting Means
The disconnecting means provided for cranes and hoists may consist of two or more lock-open
type motor circuit switches or circuit breakers. Article 610, Part IV, of the NEC, Disconnecting Means, and the installation and operating plans should be studied carefully to determine the disconnecting means requirements and locations. The two basic disconnects to consider are:
1. The runway conductor (conductors run along a crane runway for power or control) disconnect shall7.5 be installed in accordance with NEC 610.31; and
2. The crane and hoist disconnect that shall7.6 be provided in the leads from the runway contact conductors or other power supply in accordance with NEC 610.32.
An additional control switch or a remote control switch is required7.6 if the second disconnecting means is not accessible to the operator (see Fig. 7-1). A monorail hoist does not require7.6 a disconnecting means in the leads to the hoist machinery if it is controlled from the floor, as long as it is within view of the power supply disconnect, and if there is no work platform provided to service the hoist machinery (see Fig. 7-2).
Fig. 7-1. Additional control switch or a remote control.
Fig. 7-2. Control switch without second disconnect.
7.2.3 Grounding
NEC grounding requirements consider the crane or hoist with all its associated equipment, including electrical equipment, as a single piece of equipment. Therefore, all the conductive component parts shall7.7 be bonded together so that the entire crane or hoist is grounded in compliance with NEC Article 250, and NEC Article 610. Metal-to-metal contact is required between all surfaces, including the trolley wheels and bridge. If any such surfaces are painted or otherwise insulated, a separate bonding conductor is required.
The trolley frame and bridge frame shall7.7 not be considered as electrically grounded through the bridge and trolley wheels and its respective tracks. A separate bonding conductor shall7.7 be provided.
The bonding of all conductive surfaces by metal-to-metal contact is not to be considered as the EGC for the electrical equipment (motors, motor controllers, lighting fixtures, transformers, etc.) on the crane or hoist. The equipment ground conductors that are run with the circuit conductors shall7.8 comply with NEC Article 250.
7.2.4 Control
A limit switch is required7.9 to prevent the load block from passing the safe upper travel limit on all hoisting mechanisms.
7.2.5 Clearances
In the direction of live parts, the working space clearance is 2.5 feet, and doors enclosing live parts that may require service or maintenance shall7.10 open at least 90 degrees or be removable.
7.2.6 OSHA and NEC Requirements
29 CFR 1910.179 and NEC Article 610, Part F, provide additional electrical requirements derived from ANSI and other standards. Significant requirements are the following:
1. Control circuit voltage shall7.11 not exceed 600 V AC or DC. Pendant pushbutton voltage shall7.12 not exceed 150 V AC or 300 V DC.
2. Support shall7.13 be provided for pendant multiconductor cables.
3. Electrical systems for cranes and hoists shall7.14 provide a fail-safe operation. When power fails, all motors shall7.15 be automatically disconnected so that they do not resume operation when the power comes back on. Automatic cranes shall7.14 not commence motion automatically when the power comes on after an outage. Pendant pushbuttons shall7.16 be returned to the off position when pressure is released. When the signal from a remote controller fails, all motion shall7.17 stop.
7.2.7 Maintenance and Operations
It is important to have a comprehensive electrical maintenance program for cranes and hoists.
Every electrical part and circuit plays a critical operational safety role and shall7.18 be checked and serviced at the frequency and in the manner specified by OSHA, CMAA, ANSI, as well as the manufacturer's manual. Required weekly, monthly, and semiannual tests and required record-keeping are contained in ANSI B-30 and CMAA documents.
The basic references for safe operation and maintenance of cranes and hoists are contained in sections of 29 CFR 1910.179 and 29 CFR 1926.1501.
7.2.8 Documented Maintenance
Maintenance checklists and schedules in compliance with OSHA, owner's manuals, and manufacturer's requirements for the specific equipment shall7.18 be provided, as required.
Weekly, monthly, and semiannual inspections shall7.19 be conducted, and comments and condition of the inspected part shall7.20 be documented.
The recommended frequencies of inspections vary in accordance with application, usage, and authority. Frequent inspection and periodic inspection are defined by OSHA as daily to monthly and 1 to 12 months, respectively. Typical inspection frequencies for electrical equipment of cranes and hoists are as shown in Table 7-1.
Weekly Monthly Semiannually
Brakes Control Operations Motors
Pushbuttons Collectors Control Panel
Master Resistors Switch Conductors Mainline
Disconnect Warning
Device
Table 7-1. Inspection frequencies for components of cranes and hoists.
The inspection records should provide an ongoing safety assessment of the equipment and be used to predict repair-part replacement. The inspector should date and initial all inspections.
7.2.9 Mechanical Elevating and Rotating Equipment
The primary electrical safety concern is working in proximity to live and unguarded electrical overhead lines by un-insulated equipment. Unless these lines are visibly grounded at the point of work and the owner of the lines indicates they are de-energized, barriers or insulating protective material shall7.21 be installed to prevent worker contact with them. The following clearances shall7.22 be maintained between equipment and electrical overhead lines:
1. Lines 50 kV or below: 10 feet between the lines and any part of the equipment or load, except when in transit; and
2. Lines over 50 kV: 10 feet plus 0.4 inch for every 1 kV above 50 kV.
In locations and situations where it is possible the operator may have difficulty observing that these clearances are being maintained, someone shall7.23 be designated to monitor the clearances and provide the operator with timely warning before contact can be made. The use of cage-type boom guards, insulating links, or a proximity sensor should not alter the electrical safety requirements of 29 CFR 1910.269(p)(4) and 29 CFR 1926.550, even if these devices are required. (See Fig. 7-3.)
Fig. 7-3. Clearance distances between equipment and overhead lines.